CHARACTERIZATION OF GENETIC ELEMENTS REQUIRED FOR SITE-SPECIFIC INTEGRATION OF THE TEMPERATE LACTOCOCCAL BACTERIOPHAGE PHI-LC3 AND CONSTRUCTION OF INTEGRATION-NEGATIVE PHI-LC3 MUTANTS

The genetic elements required for the integration of the temperate lactococcal bacteriophage phiLC3 into the chromosome of its bacterial host, Lactococcus lactis subsp. cremoris, were identified and characterized. The phiLC3 phage attachment site, attP, was mapped and sequenced. DNA sequence analysis of attP and of the bacterial attachment site, attP, as well as the two phage-host junctions, attR and attL, in the chromosome of a phiLC3 lysogen, identified a 9-bp common core region, 5'-TTCTTCATG'-3, within which the strand exchange reaction takes place during integration. The attB core sequence is located within the C-terminal part of an open reading frame of unknown function. The phiLC3 integrase gene (int), encoding the phiLC3 site-specific recombinase, was identified and is located adjacent to attP. The phiLC3 Int protein, as deduced from the nucleotide sequence, is a basic protein of 374 amino acids that shares significant sequence similarity with other site-specific recombinases of the integrase family. Phage phiLC3 int- and int-attP-defective mutants, conferring an abortive lysogenic phenotype, were constructed.